Apparatus for producing water-absorbing resin particles

ABSTRACT

An apparatus for producing water-absorbing resin particles for which surface cross-linking treatment is conducted, the surface cross-linking treatment being conducted by spraying a surface cross-linking agent to a water-absorbing resin particle precursor and heating the agent and the precursor, the apparatus includes a treatment container in which the surface cross-linking treatment is conducted, a stirring device including a stirring member disposed in the treatment container, a heating device that heats an inside of the treatment container; and a spray nozzle disposed in the treatment container, the spray nozzle spraying into the treatment container the surface cross-linking agent supplied from a surface cross-linking agent supply source in an exterior of the treatment container through a supply pipe. In a flow path in the spray nozzle spanning from an entrance of the spray nozzle to a spray exit, a point whose opening cross-section is smallest in a flow path through which a fluid passes is the spray exit. A product with further stable physical properties can thereby be acquired.

This application is a continuation of U.S. patent application Ser. No.16/088,653 filed on Sep. 26, 2018, which is a U.S. National Stage entryof International Application No. PCT/JP2017/005665 filed on Feb. 16,2017, which claims priority from Japanese Application No. 2016-071026filed on Mar. 31, 2016, the entire disclosure of each of the foregoingis herein incorporated by reference.

TECHNICAL FIELD

This disclosure relates to an apparatus for producing water-absorbingresin particles, that conducts surface cross-linking treatment byspraying a surface cross-linking agent to a water-absorbing resinparticle precursor and heating them to acquire surface cross-linkingtreated water-absorbing resin particles.

BACKGROUND ART

Water-absorbing resin particles are used in a wide range of fields suchas the use for sanitary materials such as sanitary products anddisposable diapers, the use for agriculture and gardening such aswater-retaining agents and agricultural ameliorants, or the use forindustrial materials such as water-stopping agents and dew-condensationpreventing agents. The water-absorbing resin particles are actively usedespecially in the fields related to the sanitary materials.

The water-absorbing resin particles used in the various types of use arelightly cross-linked high molecules and, for example, starch-basedwater-absorbing resins such as a hydrolysate of a starch-acrylonitrilegraft copolymer (Patent Document 1) and a neutralized product of astarch-acrylic acid graft copolymer (Patent Document 2), a saponifiedproduct of a vinyl acetate-acrylic acid ester copolymer (Patent Document3), and partially-neutralized products of polyacrylic acid (PatentDocument 4, Patent Document 5, and Patent Document 6) are known.

These types of water-absorbing resin particles are each acquired bypolymerizing, drying, and, when necessary, crushing and classifyingwhile the water-absorbing resin particles are also modified after thepolymerizing and the drying by adding various types of compound to theacquired water-absorbing resin particles to further impart additionalfunctions thereto. A what-is-called surface cross-linking technique isknown as a modification method for the water-absorbing resin particlesaccording to which the vicinity of the surface of the water-absorbingresin particle precursor is cross-linked by a surface cross-linkingagent. What is considered to be most important in the surfacecross-linking treatment is to uniformly surface-cross-link the surfaceof each of the water-absorbing resin particles and, to do this, uniformmixing of the water-absorbing resin particle precursor and the surfacecross-linking agent with each other before the surface cross-linking isimportant.

Such methods are known as the techniques to uniformly mix thewater-absorbing resin particle precursor and the surface cross-linkingagent with each other before the cross-linking is conducted, as a methodof spraying a surface cross-linking agent in fine liquid droplets tothereby bring the surface cross-linking agent in contact with awater-absorbing resin powder (Patent Document 7), a method of sprayingin an empty circular cone-shape showing an annular shape or in anelliptic cone-shape showing a double convex lens-shape (Patent Document8), and a method of spraying a liquid onto water-absorbing polymerparticles using a spray nozzle (Patent Document 9).

PATENT DOCUMENTS

-   Patent Document 1: Japanese Patent Publication No. 49-43395-   Patent Document 2: Japanese Laid-Open Patent Publication No.    51-125468-   Patent Document 3: Japanese Laid-Open Patent Publication No.    52-14689-   Patent Document 4: Japanese Laid-Open Patent Publication No.    62-172006-   Patent Document 5: Japanese Laid-Open Patent Publication No.    57-158209-   Patent Document 6: Japanese Laid-Open Patent Publication No.    57-21405-   Patent Document 7: Japanese Laid-Open Patent Publication No.    4-246403-   Patent Document 8: Japanese Laid-Open Patent Publication No.    2002-201290-   Patent Document 9: International Patent Publication No. 2010-521578

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

A problem however arises that a product with constant physicalproperties is not always stably acquired with the methods andapparatuses each for producing the modified water-absorbing resinparticles by adding the various types of liquid substance to thewater-absorbing resin particles using the spray nozzle as above in acommercial plant or the like.

An object of this disclosure is to provide an apparatus for producingwater-absorbing resin particles from which a product with further stablephysical properties can be acquired, as an apparatus for producingwater-absorbing resin particles by conducting surface cross-linkingtreatment, the treatment being conducted by spraying a surfacecross-linking agent to a water-absorbing resin particle precursor andheating the agent and the precursor.

Means for Solving the Problems

An apparatus for producing water-absorbing resin particles according toan aspect of this disclosure is an apparatus for producingwater-absorbing resin particles for which surface cross-linkingtreatment is conducted, the surface cross-linking treatment beingconducted by spraying a surface cross-linking agent to a water-absorbingresin particle precursor and heating the agent and the precursor, theapparatus is configured to include a treatment container for the surfacecross-linking treatment to be conducted therein, a stirring device thatincludes a stirring member disposed in the treatment container, aheating device that heats the inside of the treatment container, and aspray nozzle that is disposed in the treatment container and that spraysinto the inside of the treatment container a surface cross-linking agentsupplied from a surface cross-linking agent supply source in theexterior of the treatment container through a supply pipe, wherein, in aflow path in the spray nozzle spanning from an entrance of the spraynozzle to a spray exit, a point whose opening cross-section is smallestin a flow path through which a fluid passes is the spray exit.

Effect of the Invention

According to this disclosure, a product with further stable physicalproperties can be acquired using the apparatus for producingwater-absorbing resin particles that conducts the surface cross-linkingtreatment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart of production steps for water-absorbing resinparticles according to an embodiment of this disclosure.

FIG. 2 is a configuration diagram of a treating apparatus of theembodiment.

FIG. 3A is a cross-sectional diagram of an assembly state of a spraynozzle included in the treating apparatus of the embodiment.

FIG. 3B is an exploded cross-sectional diagram of the spray nozzleincluded in the treating apparatus of the embodiment.

FIG. 4A is a cross-sectional diagram of an assembly state of a spraynozzle of a comparative embodiment.

FIG. 4B is an exploded cross-sectional diagram of the spray nozzle ofthe comparative embodiment.

FIG. 5 is a configuration diagram of a treating apparatus according to amodification example of the embodiment.

EMBODIMENT(S) FOR CARRYING OUT THE INVENTION

(Findings to be Basis of this Disclosure)

The inventors of this application intensively studied to solve theproblem and, as a result, focused on a spray method for a surfacecross-linking agent in their search for the cause of the fact that anyproduct with constant physical properties was unable to stably beacquired. The findings found by the inventors of this application willbe described as follows.

When a spray nozzle is used to spray a surface cross-linking agent, astrainer (a filter) is traditionally arranged in the spray nozzle toprotect the spray nozzle, that is, to prevent any nozzle clogging.

On the other hand, when surface cross-linking treatment by mixing asurface cross-linking agent with a water-absorbing resin particleprecursor and heating them is conducted, a trace of unreacted monomersremaining in the water-absorbing resin particle precursor vaporizes orthe like and enters the inside of the spray nozzle to adhere to thestrainer. When the heating is conducted in the surface cross-linkingtreatment, the heat is also transmitted to the strainer, polymerizationof the adhering unreacted monomers is thereby advanced, and the spraynozzle is thereby blocked out. When drying treatment (or heatingtreatment) is further conducted to reduce the water amount stillremaining in the water-absorbing resin particles after the surfacecross-linking treatment, the heat is further transmitted to thestrainer, polymerization of the adhering unreacted monomers is advanced,and the spray nozzle is blocked out. In the case where areduced-pressure drying treatment is conducted as the drying treatment,when the pressure is recovered from the reduced-pressure state to thenormal pressure, the unreacted monomers tend to enter the inside of thespray nozzle and the spray nozzle tends to be blocked out.

When this blocking out occurs in each of even some of the plural spraynozzles disposed in the apparatus for producing water-absorbing resinparticles, the spray function for the surface cross-linking agent isdegraded, and the water-absorbing resin particle precursor and thesurface cross-liking agent cannot uniformly be mixed with each other. Asa result, any product with constant physical properties cannot stably beacquired. The inventors found this fact and, based on this finding,completed this disclosure.

An apparatus for producing water-absorbing resin particles of a firstaspect of this disclosure is an apparatus for producing water-absorbingresin particles for which surface cross-linking treatment is conducted,the surface cross-linking treatment being conducted by spraying asurface cross-linking agent to a water-absorbing resin particleprecursor and heating the agent and the precursor, the apparatusincludes a treatment container for the surface cross-linking treatmentto be conducted therein, a stirring device that includes a stirringmember disposed in the treatment container, a heating device that heatsthe inside of the treatment container, and a spray nozzle that isdisposed in the treatment container and that sprays into the treatmentcontainer a surface cross-linking agent supplied from a surfacecross-linking agent supply source in the exterior of the treatmentcontainer through a supply pipe, wherein the apparatus is configuredsuch that, in a flow path in the spray nozzle spanning from an entranceof the spray nozzle to a spray exit, a point whose opening cross-sectionis smallest in a flow path through which a fluid passes is the sprayexit.

According to this configuration, any fine-mesh member whose openingcross-section is finely divided such as a strainer and any portionhaving the flow path itself narrowed to be thin are not disposed in thecourse of the flow path that spans from the entrance of the spray nozzleto the spray exit. Occurrence of any clogging can thereby be suppressedin a portion of the flow path other than the spray exit of the spraynozzle even when the case occurs where, in the treatment container, atrace of unreacted monomers and the like remaining in thewater-absorbing resin particle precursor enters from the spray exit ofthe spray nozzle during discontinuation of the spraying. When thesurface cross-linking agent is sprayed, the spray exit of the spraynozzle has a relatively high spray pressure applied thereto compared tothe other portion in the spray nozzle and any adhering object can beblown off.

An apparatus for producing water-absorbing resin particles of a secondaspect of this disclosure is the apparatus for producing water-absorbingresin particles of the first aspect, wherein a strainer including pluralopenings each having an opening cross-section smaller than the openingcross-section of the spray exit of the spray nozzle is disposed in thesupply pipe on the upstream side of the entrance of the spray nozzle.

According to this configuration, the strainer can prevent supply of anyforeign object (that is, any foreign object that may clog the sprayexit) to the spray nozzle through the supply pipe for the surfacecross-linking agent. The strainer to protect the spray exit is disposednot in the spray nozzle but in the course of the supply pipe on theupstream side of the spray nozzle. Protection of the spray exit canthereby be facilitated by the strainer suppressing occurrence of anyclogging in the spray nozzle by the entrance of the unreacted monomersand the like.

An apparatus for producing water-absorbing resin particles of a thirdaspect of this disclosure is the apparatus for producing water-absorbingresin particles of the second aspect, wherein the heating device is aheating jacket arranged on the outer circumference of the treatmentcontainer, and wherein the strainer is disposed in the supply pipe onthe outer side of the heating jacket.

According to this configuration, in the supply pipe, the strainer isdisposed in the portion thereof that tends to avoid any influence by theheat of the heating jacket. Protection of the spray exit can thereby befacilitated by the strainer suppressing occurrence of any clogging inthe spray nozzle by the entrance of the unreacted monomers.

An apparatus for producing water-absorbing resin particles of a fourthaspect of this disclosure is the apparatus for producing water-absorbingresin particles of the second or the third aspect, wherein the supplypipe for the surface cross-linking agent includes a first pipe that is aheader pipe having plural spray nozzles connected thereto and a secondpipe that connects the surface cross-linking agent supply source and thefirst pipe to each other, wherein the first pipe and the plural spraynozzles are arranged inside the treatment container, and wherein thestrainer is disposed in a portion on the outer side of the treatmentcontainer, of the second pipe.

According to this configuration, the strainer is disposed in the secondpipe that tends to avoid any influence by the heat from the treatmentcontainer. Protection of the spray exit can thereby be facilitated bythe strainer suppressing occurrence of any clogging in the spray nozzleby the entrance of the unreacted monomers.

An apparatus for producing water-absorbing resin particles of a fifthaspect of this disclosure is the apparatus for producing water-absorbingresin particles of any one of the second to the fourth aspects, furtherincluding a cooling device that cools the strainer.

According to this configuration, any thermal reaction for the unreactedmonomers can be suppressed by the cooling of the strainer by the coolingdevice. Even when the case occurs where the unreacted monomers adhere tothe strainer, advancement of any polymerization of the unreactedmonomers can thereby be suppressed and occurrence of any clogging in thestrainer can be suppressed.

EMBODIMENT

An embodiment according to this disclosure will be described below indetail with reference to the drawings while the scope of this disclosureis not bound by the description and, to items other than the followingexemplification, proper changes can be made for implementation withoutdiscrediting the gist of this disclosure.

[Production Steps for Water-Absorbing Resin Particles]

FIG. 1 depicts a flowchart of production steps for water-absorbing resinparticles according to an embodiment of this disclosure. As depicted inFIG. 1, the water-absorbing resin particles are prepared at a resinparticle preparation step s15 that includes at least a polymerizationstep s11, a condensation step s12, a surface cross-linking step s13, anda drying step s14.

<Polymerization Step>

The polymerization step s11 is a step at which the water-absorbing resinparticles are acquired by polymerization-reacting water-solubleethylenic unsaturated monomers. The polymerization method for thewater-soluble ethylenic unsaturated monomers is not especially limited,and an aqueous solution polymerization method, an emulsionpolymerization method, a reverse phase suspension polymerization method,or the like is used that are typical polymerization methods.

In the aqueous solution polymerization method, the polymerization isconducted by, for example, heating a water solution of the water-solubleethylenic unsaturated monomers, an internal cross-linking agent, and awater-soluble radical polymerization initiator, stirring these whennecessary. In the aqueous solution polymerization method, water ishandled as a liquid medium and the water-soluble ethylenic unsaturatedmonomers are established as a solution for the polymerization reactionto take place.

In the reverse phase suspension polymerization method, thepolymerization is conducted by, for example, heating a water solution ofthe water-soluble ethylenic unsaturated monomers, a surfactant, ahydrophobic high molecule-based dispersion agent, a water-solubleradical polymerization initiator, and an internal cross-linking agent,stirring these in a petroleum-based hydrocarbon dispersion medium. Inthe reverse phase suspension polymerization method, water and thepetroleum-based hydrocarbon dispersion medium are handled as the liquidmedia, and the polymerization reaction takes place with the watersolution of the water-soluble ethylenic unsaturated monomers added tothe petroleum-based hydrocarbon dispersion medium to be in a suspensionstate.

In the following, as an example of the embodiment of this disclosure, amethod of producing the water-absorbing resin particles using thereverse phase suspension polymerization method will be described, thatenables precise control of the polymerization reaction and control ofthe particle diameter in a wide range.

Examples of the water-soluble ethylenic unsaturated monomer used as theraw material of the water-absorbing resin particles include, forexample, monomers each having an acid radical such as (meth)acrylicacid, 2-(meth)acrylamide-2-methylpropanesulfonic acid, maleic acid, andtheir salts; non-ionic unsaturated monomers such as (meth)acrylamide,N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, andN-methylol(meth)acrylamide; and amino-group-containing unsaturatedmonomers such as diethylaminoethyl(meth)acrylate,diethylaminopropyl(meth)acrylate and their quaternized products. Any oneof these may be used alone, or two or more thereof may be used together.“(meth)acryl” means “acryl” and “methacryl”.

Examples of the alkaline compound used when a monomer having an acidradical is neutralized to be a salt include compounds each of lithium,sodium, potassium, ammonium, and the like. Examples thereof include, forexample, sodium hydroxide, potassium hydroxide, lithium hydroxide,sodium carbonate, and ammonium bicarbonate.

In this embodiment, the water-soluble ethylenic unsaturated monomers areused as a water solution. It is preferred that the monomer concentrationof the water-soluble ethylenic unsaturated monomer water solution being20% by mass to the saturation concentration.

The water-soluble ethylenic unsaturated monomer water solution mayinclude, when necessary, a chain transfer agent, a thickener, and thelike. Examples of the chain transfer agent include, for example,compounds such as thiols, thiol acids, secondary alcohols,hypophosphorous acid, and phosphorous acid. Any one of these may be usedalone, or two or more thereof may be used together. Examples of thethickener include carboxymethyl cellulose, hydroxyethyl cellulose,hydroxypropyl cellulose, methyl cellulose, polyethylene glycol,polyacrylic acid, neutralized products of polyacrylic acid,polyacrylamide, and the like.

Examples of the petroleum-based hydrocarbon dispersion medium include,for example, aliphatic hydrocarbons each having 6 to 8 carbon atoms suchas n-hexane, n-heptane, 2-methylhexane, 3-methylhexane,2,3-dimethylpentane, 3-ethylpentane, and n-octane, alicyclichydrocarbons such as cyclohexane, methylcyclohexane, cyclopentane,methylcyclopentane, trans-1,2-dimethylcyclopentane,cis-1,3-dimethylcyclopentane, and trans-1,3-dimethylcyclopentane, andaromatic hydrocarbons such as benzene, toluene, and xylene. Among these,from the viewpoints of industrially easy availability and the safety,those more advantageously used are the aliphatic hydrocarbons eachhaving 6 to 8 carbon atoms such as n-hexane, n-heptane, 2-methylhexane,3-methylhexane, and n-octane; and alicyclic hydrocarbons each having 6to 8 carbon atoms such as cyclohexane, methylcyclopentane, andmethylcyclohexane. Any one of these petroleum-based hydrocarbondispersion media may be used alone, or two or more thereof may be usedtogether.

In the reverse phase suspension polymerization, the water-solubleethylenic unsaturated monomer water solution is dispersed in thepetroleum-based hydrocarbon dispersion medium and, to acquire furtherstable polymerized particles, the surfactant and, when necessary, thehydrophobic high molecule-based dispersion agents are used. From theviewpoint that the polymerization is stably completed, when thesurfactant and the hydrophobic high molecule-based dispersion agent canbe caused to be present before the water-soluble ethylenic unsaturatedmonomer water solution is polymerized, the water-soluble ethylenicunsaturated monomer water solution can thereby be sufficiently dispersedin the petroleum-based hydrocarbon dispersion medium, and the liquiddroplets thereof can be stabilized to thereafter conduct thepolymerization, the timing to add each of the above is not especiallylimited. In general, the surfactant and the hydrophobic highmolecule-based dispersion agent are solved or dispersed in advance inthe petroleum-based hydrocarbon dispersion medium before the addition ofthe water-soluble ethylenic unsaturated monomer water solution.

Examples of the surfactant used to maintain the dispersion stability forthe polymerization include, for example, non-ionic surfactants such assorbitan fatty acid ester, polyoxyethylenesorbitan fatty acid ester,polyglycerin fatty acid ester, polyoxyethyleneglycerin fatty acid ester,sucrose fatty acid ester, sorbitol fatty acid ester,polyoxyethylenesorbitol fatty acid ester, polyoxyethylenealkyl ether,polyoxyethylenealkylphenyl ether, polyoxyethylene castor oil,polyoxyethylene hydrogenated castor oil, alkylallyl formaldehydecondensed polyoxyethylene ether, polyoxyethylenepolyoxypropylalkylether, polyethylene glycol fatty acid ester, alkylglucoside,N-alkylgluconamide, polyoxyethylene fatty acid amide, andpolyoxyethylenealkylamine; and anionic surfactants such as fatty acidsalts, alkylbenzenesulfonic acid salts, alkylmethyltaurine acid salts,polyoxyethylenealkylphenyl ether sulfate ester salts,polyoxyethylenealkyl ether sulfate ester salts, polyoxyethylenealkylether sulfate and its salts, polyoxyethylenealkylphenyl ether phosphoricacid and its salts, and polyoxyethylenealkyl ether phosphoric acid andits salts. Any one of these may be used alone, or two or more thereofmay be used together.

The hydrophobic high molecule-based dispersion agent may be usedtogether with the surfactant to further enhance the dispersion stabilityfor the polymerization. It is preferred that the hydrophobic highmolecule-based dispersion agent capable of being solved or dispersed inthe used petroleum-based hydrocarbon dispersion medium be selected to beused, and examples thereof include, for example, those each having theviscosity-average molecular weight of 20,000 or smaller, preferably10,000 or smaller, and further preferably 5,000 or smaller. Examplesthereof include, for example, maleic acid anhydride-modifiedpolyethylene, maleic acid anhydride-modified polypropylene, maleic acidanhydride-modified ethylene-propylene copolymer, maleic acidanhydride-ethylene copolymer, maleic acid anhydride-propylene copolymer,maleic acid anhydride-ethylene-propylene copolymer, polyethylene,polypropylene, ethylene-propylene copolymer, oxidized polyethylene,oxidized polypropylene, oxidized ethylene-propylene copolymer,ethylene-acrylic acid copolymer, ethylcellulose, polybutadiene maleicanhydride, ethylene-propylene-diene ternary copolymer (EPDM) maleicanhydride, and the like.

When the water-soluble ethylenic unsaturated monomer water solution isadded to the petroleum-based hydrocarbon dispersion medium that fills inadvance the polymerization reactor, to be dispersed therein, thedispersion is conducted with a stirring means while the conditions forthe stirring with this stirring means differ depending on the desireddispersion liquid droplet diameter and cannot generally be determined.The dispersion liquid droplet diameter can be adjusted using the type,the blade diameter, and the number of rotations of the stirring blade ofthe stirring means and the like. For example, a propeller blade, apaddle blade, an anchor blade, a turbine blade, a pfaudler blade, aribbon blade, a full-zone blade (manufactured by Kobelco Pantech Co.,Ltd.), a Maxblend blade (manufactured by Sumitomo Heavy Industries,Ltd.), and SuperMix (manufactured by Satake Chemical Equipment Mfg.,Ltd.) can be used as the stirring blade.

In the polymerization reactor, the water-soluble ethylenic unsaturatedmonomer water solution added at a predetermined addition rate to thepetroleum-based hydrocarbon dispersion medium is fully stirred to bedispersed by the stirring means in the petroleum-based hydrocarbondispersion medium in the presence of the surfactant to thereby stabilizethe droplets. After the inside of the polymerization reactor issufficiently substituted by nitrogen, reverse phase suspensionpolymerization is conducted using the water-soluble radicalpolymerization initiator in the presence of, when necessary, theinternal cross-linking agent to acquire a suspension of a hydrogelcross-linked polymer (hereinafter, referred to as “water-absorbing resinparticle precursor”).

Examples of the water-soluble radical polymerization initiator used inthis embodiment include, for example, persulfates such as potassiumpersulfate, ammonium persulfate, and sodium persulfate; peroxides suchas hydrogen peroxide; and azo compounds such as2,2′-azobis(2-amidinopropane) dihydrochloride,2,2′-azobis[N-(2-carboxyethyl)-2-methylpropiondiamine] tetrahydrate,2,2′-azobis(1-imino-1-pyrrolidino-2-methylpropane) dihydrochloride, and2,2′-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide].

The water-soluble radical polymerization initiator may be used as aredox polymerization initiator by being used together with a reducingagent such as a sulfite or ascorbic acid.

Examples of the internal cross-linking agent used when necessaryinclude, for example, polyols such as (poly)ethylene glycol [“(poly)”means the case where the prefix “poly” is present and the case theprefix is absent, and the same is hereinafter applied], 1,4-butanediol,glycerin, and trimethylolpropane, polyunsaturated esters each having twoor more vinyl groups and each acquired by reacting a polyol and anunsaturated acid such as acrylic acid or methacrylic acid,bisacrylamides such as N,N′-methylenebisacrylamide, and polyglycidylcompounds each having two or more glycidyl groups such as(poly)ethyleneglycoldiglycidyl ether, (poly)ethyleneglycoltriglycidylether, (poly)glycerindiglycidyl ether, (poly)glycerintriglycidyl ether,(poly)propyleneglycolpolyglycidyl ether, and (poly)glycerolpolyglycidylether. Any one of these may be used alone, or two or more thereof may beused together.

The reaction temperature for the reverse phase suspension polymerizationin the polymerization reactor differs depending on the type and theamount of the used polymerization initiator and cannot generally bedetermined while the reaction temperature is preferably 30 to 120° C.and more preferably 40 to 100° C. The reaction temperature is 30° C. orhigher and any reduction of the polymerization rate can thereby besuppressed. The reaction temperature is 120° C. or lower and occurrenceof any abrupt polymerization reaction can thereby be suppressed.

The polymerization reaction liquid including the water-absorbing resinparticle precursor, acquired as above (the suspension liquid of thewater-absorbing resin particle precursor) is polymerized as a“first-stage polymerization”, and “multi-stage polymerization” ofrepeating polymerization for some times each by adding the water-solubleethylenic unsaturated monomer water solution may thereafter beconducted.

From the viewpoint of acquisition of a proper agglomeration particlediameter in the multi-stage polymerization, the size of the particleacquired by the polymerization of the water-soluble ethylenicunsaturated monomers in the first-stage is, as the median particlediameter, preferably 20 to 200 μm, more preferably 30 to 150 μm, andfurther preferably 40 to 120 μm.

When two-stage polymerization is conducted, the particles acquired inthe first-stage polymerization are agglomerated and water-absorbingresin particles can thereby be acquired whose average particle diameteris relatively large and that are suitable for use for a sanitarymaterial. The size of the agglomeration particle of the water-absorbingresin particles advantageous for the use for a sanitary material ispreferably 200 to 600 μm, further preferably 250 to 500 μm, and mostpreferably 300 to 450 μm.

The water-soluble ethylenic unsaturated monomer same as any of thoseexemplified as the water-soluble ethylenic unsaturated monomer for thefirst-stage polymerization is usable as the water-soluble ethylenicunsaturated monomer for the second-stage polymerization while the typeof the monomer, the degree of neutralization, the neutralized salt, andthe concentration of the monomer water solution may be same/equal as/tothose of the water-soluble ethylenic unsaturated monomer for thefirst-stage polymerization or may be different therefrom.

Any polymerization initiator is usable that is selected from thoseexemplified for the water-soluble ethylenic unsaturated monomer watersolution for the first-stage polymerization as the polymerizationinitiator added to the water-soluble ethylenic unsaturated monomer watersolution in the second-stage polymerization.

The internal cross-linking agent, the chain transfer agent, and the likemay be added when necessary to the water-soluble ethylenic unsaturatedmonomer water solution for the second-stage polymerization, and anyinternal cross-linking agent, any chain transfer agent, and the like areusable that are selected from those exemplified for the water-solubleethylenic unsaturated monomer water solution for the first-stagepolymerization.

The reaction temperature for the reverse phase suspension polymerizationin the second stage also differs depending on the type and the amount ofthe polymerization initiator and cannot generally be determined whilethe reaction temperature is preferably 30 to 120° C. and more preferably40 to 100° C. When the multi-stage polymerization in two or more stagesis conducted, the multi-stage polymerization can be conducted byhereinafter reading “the second-stage polymerization” as the third-stagepolymerization or the fourth-stage polymerization.

<Condensation Step>

The condensation step s12 is the step of condensing the polymerizationreaction liquid that is acquired at the polymerization step s11 bydistilling away the liquid component from the polymerization reactionliquid to acquire the water-absorbing resin particle precursor.

The distilling treatment for the liquid component from thepolymerization reaction liquid at the condensation step s12 may beconducted under the normal pressure or a reduced pressure, or may beconducted in a flow of a gas such as nitrogen to increase the efficiencyof the distilling of the liquid component.

When the distilling treatment for the liquid component from thepolymerization reaction liquid is conducted at the normal pressure, theset temperature for the condensation is preferably 70 to 250° C., morepreferably 80 to 180° C., further preferably 80 to 140° C., and mostpreferably 90 to 130° C. When the distilling treatment for the liquidcomponent from the polymerization reaction liquid is conducted at areduced pressure, the set temperature for the condensation is preferably60 to 100° C. and more preferably 70 to 90° C.

<Surface Cross-Linking Step>

The surface cross-linking step s13 is the step of adding the surfacecross-linking agent having two or more functional groups each having thereactive property for a functional group derived from the water-solubleethylenic unsaturated monomer, to the water-soluble resin particleprecursor acquired after the distilling treatment for the liquidcomponent from the polymerization reaction liquid, and therebyincreasing the cross-link density in the surface layer of thewater-soluble resin particle precursor to acquire the water-solubleresin particles. The water-soluble resin particles whose surface layerseach having the high cross-link density have various types of highperformance such as the absorption capacity under load, thewater-absorbing rate, and the gel strength, and have advantageousperformance for the use for a sanitary material.

The surface cross-linking agent used in the cross-linking reaction isnot especially limited only when the surface cross-linking agent canreact with a functional group that is derived from the water-solubleethylenic unsaturated monomer used in the polymerization.

Examples of the used surface cross-linking agent include, for example,polyols such as ethylene glycol, propylene glycol, 1,4-butanediol,trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropyleneglycol, and polyglycerin; polyglycidyl compounds such as(poly)ethyleneglycoldiglycidyl ether, (poly)ethyleneglycoltriglycidylether, (poly)glycerindiglycidyl ether, (poly)glycerintriglycidyl ether,(poly)propyleneglycolpolyglycidyl ether, and glycerolpolyglycidyl ether;haloepoxy compounds such as epichlorohydrin, epibromohydrin, andα-methylepichlorohydrin, compounds each having two or more reactivefunctional groups such as isocyanate compounds such as2,4-tolylenediisocyanate and hexamethylenediisocyanate; oxetanecompounds such as 3-methyl-3-oxetanem ethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanem ethanol, 3-methyl-3-oxetaneethanol,3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol, oxazolinecompounds such as 1,2-ethylenebisoxazoline, and carbonate compounds suchas ethylene carbonate. Any one of these may be used alone, or two ormore thereof may be used together.

Among these, from the viewpoint of excellent reactivity, thepolyglycidyl compounds are advantageously used such as(poly)ethyleneglycoldiglycidyl ether, (poly)ethyleneglycoltriglycidylether, (poly)glycerindiglycidyl ether, (poly)glycerintriglycidyl ether,(poly)propyleneglycolpolyglycidyl ether, and (poly)glycerolpolyglycidylether.

The addition amount of the surface cross-linking agent is preferably0.01 to 5 parts by mass and more preferably 0.02 to 3 parts by massrelative to 100 parts by mass of the total amount of the water-solubleethylenic unsaturated monomers supplied in the polymerization. Theaddition amount of the surface cross-linking agent is 0.01 part by massor larger and the absorption capacity under load, the water-absorbingrate, the gel strength, and the like of the acquired water-absorbingresin can thereby be enhanced. The addition amount is 5 parts by mass orsmaller, and excessive degradation of the water-absorbing performancecan thereby be suppressed.

The surface cross-linking agent may be added as it is or may be added asa water solution as the addition method for the surface cross-linkingagent or, when necessary, may be added as a solution using a hydrophilicorganic solvent as the solvent. Examples of the hydrophilic organicsolvent include, for example, lower alcohols such as methyl alcohol,ethyl alcohol, n-propyl alcohol, isopropyl alcohol, and propyleneglycol, ketones such as acetone and methylethylketone, ethers such asdiethyl ether, dioxane, and tetrahydrofuran, amides such asN,N-dimethylformaldehyde, and sulfoxides such as dimethylsulfoxide. Anyone of these hydrophilic organic solvents may be used alone, or two ormore thereof may be used together.

The surface cross-linking reaction is preferably conducted in thepresence of water in a range of 1 to 200 parts by mass, furtherpreferably conducted in the presence of water in a range of 5 to 100parts by mass, and further more preferably conducted in the presence ofwater of 10 to 50 parts by mass relative to 100 parts by mass of thewater-absorbing resin during the distilling treatment of the liquidcomponent from the polymerization reaction liquid. The surfacecross-linking in the surface layer of each of the particles of thewater-absorbing resin can more advantageously be conducted and excellentwater-absorbing performance can be developed by adjusting the wateramount during the addition of the surface cross-liking agent as above.

The temperature for the surface cross-linking reaction is preferably 50to 250° C., more preferably 60 to 180° C., further preferably 60 to 140°C., and most preferably 70 to 120° C.

The water-absorbing resin particles acquired after undergoing thepolymerization step s11, the condensation step s12, and the surfacecross-linking step s13 as above are usually in a hydrogel state. Thedrying treatment may be conducted after the surface cross-linking steps13 when necessary, to reduce the water rate of the water-absorbingresin particles.

<Drying Step>

The case will be described where the drying step is conducted after thesurface cross-linking step s13. The drying step s14 is the step ofdrying the water-absorbing resin particles in the hydrogel state that isacquired at the surface cross-linking step s13.

Various types of method can be employed as the drying treatment methodconducted at the drying step S14 such as a drying by heating, hot airdrying, reduced pressure drying, IR ray drying, microwave drying,dewatering by azeotropy with a hydrophobic organic solvent, and highhumidity drying using high temperature steam, to establish the aimedwater content ratio, and the drying treatment method is not especiallylimited. When the drying treatment is conducted by hot air drying, thedrying treatment is conducted at a temperature (the temperature of thehot air) in a range of usually 60 to 250° C., preferably 100 to 220° C.,and more preferably 120 to 200° C. The drying time period is dependenton the surface area and the water content ratio of the polymer, and thetype of the drying machine and is selected to establish the aimed watercontent ratio. For example, the drying time period is properly selectedin a range of 1 minute to 5 hours. When the reduced pressure drying isconducted, the drying treatment is conducted at a vacuum pressure (theabsolute pressure) in a range of preferably 5 to 90 kPa and morepreferably 10 to 60 kPa.

The water content ratio of the water-absorbing resin particles usablefor this disclosure is preferably 20% by mass or lower and morepreferably 10% by mass or lower. Excellent fluidity of thewater-absorbing resin particles can be maintained by setting the watercontent ratio of the water-absorbing resin particles to be 20% by massor lower.

The surface cross-linking step s13 and the drying step s14 can beconducted using the apparatus for producing water-absorbing resinparticles of this disclosure. The configuration of a treating apparatus1 will be described that is an example of the apparatus for producingwater-absorbing resin particles according to an embodiment of thisdisclosure, with reference to the drawings. FIG. 2 is a schematicconfiguration diagram of the main configuration of the treatingapparatus 1 of this embodiment.

As depicted in FIG. 2, the treating apparatus 1 includes a treatmentcontainer 10, a stirring device 20, and a heating device 30. Thetreating apparatus 1 may employ, for example, a conductionheat-transmitting stirring and drying device.

The treatment container 10 is a horizontal drum container thataccommodates the water-absorbing resin particle precursor and thewater-absorbing resin particles, and is structured to have heatresistance and airtightness. An input entrance 11 for thewater-absorbing resin particle precursor is disposed in the upperportion of the treatment container 10, and the input entrance 11 isconnected to a supply line (such as, for example, a pipe path) for thewater-absorbing resin particle precursor. A discharge exit 12 for thewater-absorbing resin particles for which the surface cross-linking stepand the drying step are conducted is disposed in the lower side portionof the treatment container 10, and the discharge exit 12 is connected toa discharge line (such as, for example, a pipe path) for thewater-absorbing resin particles. An opening and closing mechanism (notdepicted) such as a valve to open or close the line is disposed in eachof the supply line in the vicinity of the input entrance 11 and thedischarge line in the vicinity of the discharge exit 12.

The stirring device 20 includes plural stirring boards 21 arranged inthe treatment container 10, a rotational shaft 22 to which the pluralstirring boards 21 are attached, and a rotation driving device 23 todrive and rotate the rotational shaft 22.

The rotational shaft 22 is arranged in the treatment container 10 toextend in the horizontal direction (the longitudinal direction) of thetreatment container 10 that is the horizontal drum container, and fixeseach of the stirring boards 21 to be lined in the horizontal direction.The stirring boards 21 each have a substantially disc shape, and vanesor the like may be disposed on each of the stirring boards 21 as themembers to enhance the stirring function for the water-absorbing resinparticles and the like. The rotation driving device 23 is arranged onthe outer side of the treatment container 10, rotates the rotationalshaft 22, and thereby rotates the stirring boards 21. The rotationdriving device 23 may be adapted to rotate the rotational shaft not onlyin the forward direction but also the reverse direction, and may beadapted to vary the number of rotations of the rotational shaft.

The heating device 30 includes a heat transmitting part 31 that isarranged in the inside of each of the rotational shaft 22 and thestirring boards 21 of the stirring device 20, and a heating jacket 32arranged on the outer circumference face of the outer shell of thetreatment container 10.

The heat transmitting part 31 includes a steam flow path that is formedin the inside of the rotational shaft 22 and each of the stirring boards21. Steam is supplied to the heat transmitting part 31 through a steamline (a pipe path) that is connected to an end portion of the rotationalshaft 22, and heats the water-absorbing resin particles and the likeaccommodated in the treatment container 10 through the outer surface ofeach of the stirring boards 21.

The heating jacket 32 includes the steam flow path (a heat exchanger)arranged to surround the outer surface of the outer shell of thetreatment container 10. The steam line is connected to the heatingjacket 32, and the supplied steam heats the water-absorbing resinparticle precursor or the water-absorbing resin particles and the likethat are accommodated in the treatment container 10, through the outershell of the treatment container 10.

A control configuration (such as a steam flow adjusting valve and atemperature sensor: not depicted) conducting the control of the supplyamount of the steam to the heat transmitting part 31 and the heatingjacket 32 is disposed, and the control by the control configuration isconducted to establish the required heating temperature.

A spray device 40 spraying the surface cross-linking agent is disposedin the upper portion in the treatment container 10. The spray device 40sprays the surface cross-linking agent supplied from the surfacecross-linking agent supply source disposed outside the treatmentcontainer 10 through a supply line (a supply pipe) 41, into thetreatment container 10. The spray device 40 includes plural spraynozzles 42, a first pipe 43 to be the header pipe to which the pluralspray nozzles 42 are connected, and a second pipe 44 that connects thesurface cross-linking agent supply source and the first pipe 43 to eachother.

In this embodiment, the surface cross-linking agent supply sourceincludes a tank 2 that accommodates the surface cross-linking agent, anda pump 3 that supplies the surface cross-linking agent in the tank 2 tothe supply line 41. In this embodiment, the second pipe 44 is a pipepath spanning from the exit of the pump 3 to the connection portion tothe first pipe 43 that is the header pipe.

In the spray device 40, each of the spray nozzles 42 and the first pipe43 are arranged inside the treatment container 10, and the second pipe44 includes a portion that penetrates the outer shell of the treatmentcontainer 10 and the heating jacket 32. The case where the two spraydevices 40 are disposed is exemplified by the treating apparatus 1depicted in FIG. 2 while the number of the installed spray devices 40 isnot limited.

A pure water supply line 4 and a nitrogen supply line 5 are connected tothe supply line 41 for the surface cross-linking agent, and the surfacecross-linking agent, pure water, and nitrogen can thereby be selectivelysupplied into the treatment container 10 through the supply line 41. Avalve opening or closing the line is properly disposed in the supplyline 41.

In the treating apparatus 1 of this embodiment, a strainer 45 isdisposed in the outer side portion of the treatment container 10, of thesecond pipe 44 of the supply line 41 for the surface cross-linking agent(that is, the outer shell of the treatment container 10 and the outerside portion of the heating jacket 32). The strainer 45 has a functionof capturing any foreign object by filtering the fluid that passestherethrough to avoid supplying of any foreign object to the spraynozzles 42 (any foreign object that may clog the spray exit of each ofthe spray nozzles 42). Various types of strainer may be employed as thestrainer 45 and, for example, a Y-shaped strainer may be employed. Forexample, a metal mesh filter (not depicted) is disposed in the strainer45 as the filter for filtering any foreign object. The individual meshopening cross-section (the opening area) of the metal mesh filter is setto be smaller than the opening cross-section (the opening area) of thespray exit of the spray nozzle 42.

The method of conducting the surface cross-linking step s13 and thedrying step s14 using the treating apparatus 1 of this embodiment havingthis configuration will be described.

The water-absorbing resin particle precursor acquired at thecondensation step is input from the input entrance 11 of the treatingapparatus 1 into the inside of the treatment container 10. The inputwater-absorbing resin particle precursor is stirred by the stirringdevice 20 that includes the plural stirring boards 21 disposed in thetreatment container 10. In accordance with the drying state of thewater-absorbing resin particle precursor acquired at the condensationstep, the inside of the treatment container 10 is heated by the heatingdevice 30 (the heat transmitting part 31 and the heating jacket 32) andthe water amount of the water-absorbing resin particle precursor isthereby adjusted.

Subsequently, the solution of the surface cross-linking agent is sprayedthrough the spray nozzles 42 to the water-absorbing resin particleprecursor whose water amount is adjusted to be suitable for conductingthe surface cross-linking treatment. For example, the solution of thesurface cross-linking agent is supplied from the tank 2 thataccommodates the solution of the surface cross-linking agent to thesupply line 41 by the pump 3, and the solution of the surfacecross-linking agent is sprayed from each of the spray nozzles 42 throughthe second pipe 44 and the first pipe 43. During this spraying, thespraying is conducted stirring the water-absorbing resin particleprecursor using the stirring device 20 for the solution of the surfacecross-linking agent to uniformly be supplied to the overallwater-absorbing resin particle precursor. When the surface cross-linkingtreatment is conducted using the treating apparatus 1 of thisembodiment, the surface cross-linking agent solution is a water solutionfrom the viewpoints of the easiness of handling and the safety.

After the surface cross-linking agent solution is sprayed to thewater-absorbing resin particle precursor for a predetermined timeperiod, the spraying by the spray nozzles 42 is discontinued. It ispreferred that, after the spraying is discontinued, a gas such as air,nitrogen, or the like be sprayed to remove the surface cross-linkingagent solution remaining at the tip of each of the spray nozzles 42. Inthis embodiment, the remaining solution is removed by supplying nitrogento the spray nozzles 42 through the nitrogen supply line 5 and thesupply line 41.

The inside of the treatment container 10 is thereafter heated by theheating device 30 (the heat transmitting part 31 and the heating jacket32) and the surface cross-linking reaction of the water-absorbing resinparticle precursor is thereby facilitated to conduct the surfacecross-linking treatment. The water-absorbing resin particles acquired byundergoing this surface cross-linking treatment are usually in thehydrogel state.

Subsequently, in this embodiment, the drying treatment is continuouslyconducted in the same treatment container 10 to dry the water-absorbingresin particles in the hydrogel state.

In the drying treatment, the water-absorbing resin particles are driedat the normal pressure or a reduced pressure stirring thewater-absorbing resin particles using the stirring device 20. The dryingis desirably conducted by reducing the pressure in the treatmentcontainer 10 using a decompressing device (not depicted) to suppress orprevent any adverse influence on the water-absorbing resin particles bythe thermal history. For example, a vacuum pump may be used as thedecompressing device. The water-absorbing resin particles whose watercontent ratio is adjusted to be the desired one by the drying treatmentare discharged from the discharge exit 12 of the treating apparatus 1 tothe exterior of the treatment container 10 (that is, the dischargeline).

The discharged water-absorbing resin particles further undergo the resinparticle preparation step s15 of preparing the water-absorbing resinparticles when desired, and are packed. The resin particle preparationstep s15 may include, for example, a crushing step, a classificationstep, and a mixing step.

When the water-absorbing resin particles are produced using the aqueoussolution polymerization method, the hydrogel acquired by thepolymerization is dried and the crushing treatment and theclassification treatment are conducted for the dried substance toacquire the amorphous water-absorbing resin particle precursor.

The amorphous water-absorbing resin particle precursor acquired as aboveis input into the treatment container 10 of the treating apparatus 1,the same treatments as above are conducted therefor, and thewater-absorbing resin particles can be acquired.

The configuration of the spray nozzle 42 included by the treatingapparatus 1 of this embodiment will be described with reference to across-sectional diagram of an assembly state depicted in FIG. 3A and anexploded cross-sectional diagram depicted in FIG. 3B. The configurationof a spray nozzle 142 according to a comparative embodiment to becompared to the spray nozzle 42 of this embodiment will be describedwith reference to a cross-sectional diagram of an assembly statedepicted in FIG. 4A and an exploded cross-sectional diagram depicted inFIG. 4B.

As depicted in FIG. 3A and FIG. 3B, the spray nozzle 42 of thisembodiment includes a chip part 52 that includes a spray exit 51 at itstip, a main body part 54 that includes on its one end an entrance 53connected to the first pipe 43, and a cap part 55 that couples the chippart 52 and the main body part 54 with each other.

For example, a thread portion is disposed on the outer circumference ofthe entrance 53 of the main body part 54, and is detachablythread-connected to the first pipe 43. The entrance 53 of the main bodypart 54 has a bore equal to, for example, that of the connection portionof the first pipe 43 connected thereto. In the main body part 54, theportion having the smallest opening cross-section is the entrance 53 (anopening cross-section A1).

The end portion of the chip part 52 is connected to the other end of themain body part 54. The chip part 52 has the opening cross-section thatbecomes smaller as the position of the opening cross-section becomescloser to the spray exit 51 at the tip. An opening cross-section A2 ofthe spray exit 51 is set in accordance with the specification of thefluid to be sprayed therethrough while the opening cross-section A2 isthe smallest opening cross-section in the chip part 52. Various formsmay be employed as the shape of the spray exit 51 and, for example, theopening shape having a circular cross-section or an ellipsoidalcross-section may be employed.

The cap part 55 maintains the coupling state between the chip part 52and the main body part 54 by engaging (such as, for example,thread-engaging) with the chip part 52 and the main body part 54 fromthe outer circumference side to extend over the connection portiontherebetween.

In the spray nozzle 42 of this embodiment, a flow path 56 in the spraynozzle spanning from the entrance 53 to the spray exit 51 is formed onthe inner side of the chip part 52 and the main body part 54. Theopening cross-section A2 of the spray exit 51 is set to be smaller thanthe opening cross-section A1 of the entrance 53. Accordingly, in theflow path 56 in the nozzle, the point at which the opening cross-sectionis smallest in the flow path through which the surface cross-linkingagent solution to be a liquid passes is the spray exit 51.

On the other hand, as depicted in FIG. 4A and FIG. 4B, the spray nozzle142 of the comparative embodiment includes the chip part 52, the mainbody part 54, and the cap part 55. The chip part 52, the main body part54, and the cap part 55 are the same components as the componentsincluded by the spray nozzle 42 of this embodiment.

The spray nozzle 142 includes a strainer 150. The strainer 150 isdisposed aiming at removing any foreign object that may clog the sprayexit 51. For example, a metal mesh filter is used as the strainer 150.An opening cross-section A3 (an opening area) of an individual meshopening 152 of the metal mesh filter is set to be smaller than theopening cross-section A2 (the opening area) of the spray exit 51 of thespray nozzle 42. The strainer 150 is attached to be sandwiched betweenthe chip part 52 and the main body part 54 in a flange portion 151.

In the spray nozzle 142 of the comparative embodiment having the aboveconfiguration, in a flow path 156 in the spray nozzle spanning from theentrance 53 to the spray exit 51, the point at which the openingcross-section becomes smallest in the flow path through which thesurface cross-linking agent solution passes is not the spray exit 51 butthe individual mesh opening 152 of the strainer 150.

In this specification, the expression “in a flow path in the spraynozzle spanning from the entrance of the spray nozzle to the spray exit,the point at which the opening area becomes smallest in the flow paththrough which the fluid passes is the spray exit” refers to the factthat, in the flow path in the spray nozzle, any opening cross-sectionsmaller than the opening cross-section of the spray exit is notdisposed. When the flow path is sectioned into plural openings, theopening cross-section of the flow path is the opening cross-section ofthe individual sectioned opening. For example, when the metal meshfilter is disposed in the flow path, the opening cross-section of onemesh opening of the metal mesh filter is the opening cross-section ofthe flow path.

When the surface cross-linking treatment and the drying treatment areconducted using the treating apparatus (the apparatus for producingwater-absorbing resin particles) including the spray nozzles 142 of thecomparative embodiment, the unreacted monomers remaining in thewater-absorbing resin particles vaporizes and enters the inside of thespray nozzle 142. The entering unreacted monomers adhere to the meshopening 152 of the strainer 150 whose opening cross section is smallestin the flow path 156 in the nozzle. When the surface cross-linkingtreatment and the drying treatment are conducted, the heat is alsotransmitted to the strainer 150 and the adhering unreacted monomers arepolymerized to cause blocking out of the mesh opening of the strainer150 to occur. When this blocking out grows, blocking out of the spraynozzle 142 itself occurs over time.

When this blocking out occurs in each of even some of the plural spraynozzles disposed in the treating apparatus, the spray function for thesurface cross-linking agent is degraded and the water-absorbing resinparticle precursor and the surface cross-linking agent cannot beuniformly mixed with each other.

In contrast, the spray nozzle 42 of this embodiment has no strainerdisposed therein and, in the flow path 56 in the nozzle, the point whoseopening cross-section is smallest is the spray exit 51. The spray nozzle42 is therefore configured such that any member having fine meshesformed by finely dividing the opening cross-section such as a strainerand any portion of the flow path itself that is narrowed to be thin arenot disposed in the course of the flow path 56 in the nozzle spanningfrom the entrance 53 of the spray nozzle 42 to the spray exit 51. In thecase where the spraying is discontinued, occurrence of any clogging canthereby be suppressed in a portion of the flow path other than the sprayexit 51 of the spray nozzle 42 even when the unreacted monomers and thelike remaining in the water-absorbing resin particles enter from thespray exit 51 of the spray nozzle 42 in the treatment container. Whenthe surface cross-linking agent solution is sprayed, a spray pressurerelatively high compared to those of other portions in the spray nozzle42 is applied to the spray exit 51 of the spray nozzle 42 and anyadhering object can therefore be blown off.

A product with further stable physical properties can be acquired usingthe treating apparatus 1 that conducts the surface cross-linkingtreatment and the drying treatment for the water-absorbing resinparticles and the like.

The strainer 45 including plural openings (the individual mesh openings)each having the opening cross-section that is smaller than the openingcross-section A2 of the spray exit 51 of the spray nozzle 42 is disposedin the supply line 41 on the upstream side of the entrance 53 of thespray nozzle 42. According to this configuration, the strainer 45 canprevent supply of any foreign object (that is, any foreign object thatmay clog the spray exit 51) through the supply line 41 for the surfacecross-linking agent solution. The strainer 45 for the protection of thespray exit 51 is disposed in not the spray nozzle 42 but in the courseof the supply line 41 on the upstream side of the spray nozzle 42.Protection of the spray exit 51 by the strainer 45 can thereby befacilitated suppressing occurrence of any clogging in the spray nozzle42 by the entrance of the unreacted monomers.

The strainer 45 is disposed in the supply line 41 arranged on the outerside of the outer shell of the treatment container 10 and the heatingjacket 32. According to this configuration, the strainer 45 is disposedin the portion that tends to avoid any influence by the heat of theheating jacket 32, of the supply line 41. Protection of the spray exit51 by the strainer 45 can thereby be facilitated suppressing occurrenceof any clogging in the spray nozzle 42 by the entrance of the unreactedmonomers. The case where the strainer 45 is disposed on the downstreamside of the pump 3 is taken as the example for the treating apparatus 1of this embodiment while the strainer may be disposed on the upstreamside of the pump 3.

For example, the opening cross-section A2 of the spray exit 51 of thespray nozzle 42 is 0.4 mm² and the opening cross-section A1 of theentrance 53 of the spray nozzle 42 is 52.8 mm². For example, the openingcross-section A3 of the individual mesh opening 152 of the metal meshfilter to be the strainer 150 of the spray nozzle 142 is 0.02 mm².

FIG. 5 depicts a configuration diagram of a treating apparatus 100 thatis an example of the apparatus of producing the water-absorbing resinparticles according to a modification example of this embodiment. Forthe treating apparatus 100 of the modification example depicted in FIG.5, the components same as those of the treating apparatus 1 of theembodiment are given the same reference numerals and will not again bedescribed. The difference of the treating apparatus 100 of thismodification example from the treating apparatus 1 of the embodimentwill be described.

As depicted in FIG. 5, in the treating apparatus 100, the strainer 45 isdisposed corresponding to each of the spray devices 40, in the outerside portion of the treatment container 10, of the second pipe 44 of thespray device 40. A cooling device 110 cooling the strainer 45 isdisposed to be in contact with each of the strainers 45. The coolingdevice 110 is a heat exchanger that includes the flow path for coolingwater and is connected to the supply line for the cooling water.

According to this configuration, the strainer 45 is disposed in thesecond pipe 44 that is the portion tending to avoid any influence by theheat from the treatment container 10. Protection of the spray exit 51 bythe strainer 45 can thereby be facilitated suppressing occurrence of anyclogging in the spray nozzle 42 by the entrance of the unreactedmonomers. Any thermal reaction for the unreacted monomers can besuppressed by the cooling of the strainer 45 by the cooing device 110.Even when the unreacted monomers adhere to the strainer 45, advancementof any polymerization of the unreacted monomers can thereby besuppressed and occurrence of any clogging in the strainer 45 can therebybe suppressed.

The case where the surface cross-linking treatment and the dryingtreatment are conducted for the water-absorbing resin particle precursorin each of the treating apparatuses 1 and 100 that each are an exampleof the apparatus for producing water-absorbing resin particles is takenas the example in the description for the above embodiment while theapparatus for producing water-absorbing resin particles of thisdisclosure is applicable also to the case where only the surfacecross-linking treatment is conducted by the apparatus for producingwater-absorbing resin particles.

EXAMPLES

This disclosure will be described in detail below with reference toExamples of this disclosure while this disclosure is not limited at allto Examples below.

[Production Method for Water-Absorbing Resin Particle Precursor]

<Polymerization Step>

Production of the water-absorbing resin was conducted using the reversephase suspension polymerization method. In the production of thewater-absorbing resin, for the water-soluble ethylenic unsaturatedmonomers, a two-stage polymerization reaction was conducted using thereverse phase suspension polymerization method using the radicalpolymerization initiator in the presence of a dispersion stabilizer inthe petroleum-based hydrocarbon dispersion medium.

9,000 kg of n-heptane whose temperature was maintained at 25° C. as thepetroleum-based hydrocarbon dispersion medium and 351 kg of n-heptanesolution of 10% by mass of a polyglycerin fatty acid ester as thedispersion stabilizer (trade name: Sunsoft Q-185S, produced by TaiyoChemicals Co., Ltd.) were put in the reactor main body.

The content of the reactor main body was heated for the temperaturethereof to reach 90° C. stirring the inside of the reactor main body bythe stirring means, and the dispersion stabilizer was thereby solved.The content of the reactor main body was cooled for the temperaturethereof to reach 50° C.

On the other hand, in another container, 3,505 kg of acrylic acid watersolution of 80% by mass was added as the water-soluble ethylenicunsaturated monomers and 3,890 kg of sodium hydroxide water solution of30% by mass was dropped in droplets as the alkaline neutralizer beingcooled, to conduct neutralization for the degree of neutralization to be75% by mol of the acid radicals of the water-soluble ethylenicunsaturated monomers. 3.5 kg of potassium peroxodisulfate as the radicalpolymerization initiator, 0.7 kg of N,N′-methylenebisacrylamide as thecross-linking agent, and 1,908 kg of water were further added to besolved together and the monomers for the first-stage polymerization wasprepared as a water solution.

The overall amount of the monomer water solution for the first-stagepolymerization that was prepared in the other container as above andwhose temperature was maintained at 10° C. was added to the reactor mainbody, the temperature of the content of the reactor main body was set tobe 30° C., and the inside of the system was fully substituted bynitrogen.

The content of the reactor main body was heated for the temperaturethereof to reach 55° C. stirring the inside of the reactor main body bythe stirring means to start the polymerization. After the start of thepolymerization, the temperature of the content of the reactor main bodywas increased by the polymerization heat and, from the time point atwhich the temperature of the content reached 80° C., the polymerizationwas conducted at 80° C. for 30 minutes. The content of the reactor mainbody was thereafter cooled for the temperature thereof to reach 13° C.to acquire a reaction mixture of the first stage.

On the other hand, a monomer water solution for the second-stagepolymerization was prepared in the other container. For example, in theother container, 3,505 kg of acrylic acid water solution of 80% by masswas added as the water-soluble ethylenic unsaturated monomers and 3,890kg of sodium hydroxide water solution of 30% by mass was dropped indroplets as the alkaline neutralizer being cooled to conductneutralization for the degree of neutralization to be 75% by mol of theacid radicals of the water-soluble ethylenic unsaturated monomers. 3.5kg of potassium peroxodisulfate as the radical polymerization initiator,0.7 kg of N,N′-methylenebisacrylamide as the cross-linking agent, and1,908 kg of water were further added to be solved together and themonomer water solution for the second-stage polymerization was prepared.

The overall amount of the monomer water solution for the second-stagepolymerization that was prepared in the other container as above andwhose temperature was maintained at 13° C. was put into the reactor mainbody that accommodated the above first-stage reaction mixture, and theinside of the system was sufficiently substituted by nitrogen.

The content of the reactor main body was heated for the temperaturethereof to reach 55° C. stirring the inside of the reactor main body bythe stirring means, to start the polymerization. After the start of thepolymerization, the temperature of the content of the reactor main bodywas increased by the polymerization heat and, from the time point atwhich the temperature of the content reached 80° C., the polymerizationwas conducted at 80° C. for 30 minutes to thereafter acquire a reactionmixture of the second stage.

The reaction mixture of the second stage was transferred to a condensingdevice, and the content of the condensing device was heated for thetemperature thereof to be 90° C. stirring the inside of the condensingdevice by the stirring means. Azeotropic distilling was conducted forreaction mixture with n-heptane and water to separate the n-heptane andthe water therefrom to return the n-heptane to the condensing device andto extract the water to the exterior of the system. The water-absorbingresin particle precursor (A) was thereby acquired.

Example 1

The water-absorbing resin particle precursor (A) acquired in theproduction example was transferred to a channel-type stirring dryer(manufactured by Nara Machinery Co., Ltd., a uniaxial paddle dryer:corresponding to the treating apparatus 1) that included a spray nozzle1/4MVV8005S303 (a one-fluid standard sector nozzle manufactured byIkeuchi Co., Ltd.: corresponding to the spray nozzle 142: no meshstrainer was disposed in the spray nozzle) and a Y-shaped strainer of100 mesh (corresponding to the strainer 45) disposed on the outer sideof the treatment container, 2.8 kg of ethyleneglycoldiglycidyl ether wassprayed thereto as the cross-linking agent using the spray nozzles (thespray pressure: 0.3 MPa), and the content in the dryer main body wasreacted at 90° C. to acquire a reaction mixture that wassurface-cross-linked. After the spraying, the spray nozzles were washedwith pure water and purge was conducted therefor using nitrogen.

The surface-cross-linked reaction mixture was further heated, and thewater and the n-heptane were thereby extracted to the exterior of thesystem to acquire dried water-absorbing resin particles (B). This step(the surface cross-linking step) was continuously conducted for 3 monthsand, when the produced and dried water-absorbing resin particles (B)were extracted from the dryer after 3 months, sampling was conducted foreach of the initial phase (when 10% of the overall amount wasdischarged), the intermediate phase (when 50% of the overall amount wasdischarged), and the final phase (when 90% of the overall amount wasdischarged) to thereby conduct measurement of the normal saline solutionretention capacity. The result of the measurement is presented in Table1.

The Y-shaped strainer was checked and adhesion of any foreign object andany polymerization product of acrylic acid were not observed. The insideof each of the spray nozzles was checked and adhesion of anypolymerization product of acrylic acid was not observed.

Comparative Example 1

The water-absorbing resin particle precursor (A) acquired in theproduction example was transferred to a channel-type stirring dryer(manufactured by Nara Machinery Co., Ltd., a uniaxial paddle dryer) thatincluded a spray nozzle 1/4MVV8005S303 with 6051-SS-100 (a one-fluidstandard sector nozzle manufactured by Ikeuchi Co., Ltd.: correspondingto the spray nozzle 142, a 100-mesh strainer incorporating a spraynozzle: corresponding to the strainer 150), 2.8 kg ofethyleneglycoldiglycidyl ether was sprayed thereto as the cross-linkingagent using the spray nozzle (the spray pressure: 0.3 MPa), and thecontent in the dryer main body was reacted at 90° C. to acquire areaction mixture that was surface-cross-linked. After the spraying, thespray nozzle was washed with pure water and purge was conducted thereforusing nitrogen.

The surface-cross-linked reaction mixture is further heated, and thewater and the n-heptane were thereby extracted to the exterior of thesystem to acquire dried water-absorbing resin particles (C). This step(the surface cross-linking step) was continuously conducted for 3 monthsand, when the produced and dried water-absorbing resin particles (C)were extracted from the dryer after 3 months, sampling was conducted foreach of the initial phase (when 10% of the overall amount wasdischarged), the intermediate phase (when 50% of the overall amount wasdischarged), and the final phase (when 90% of the overall amount wasdischarged) to thereby conduct measurement of the normal saline solutionretention capacity. The result of the measurement is presented in Table1.

The strainer incorporated in the spray nozzle was checked and adhesionof the polymerization product of acrylic acid was observed for 80% ofthe strainers.

TABLE 1 Normal Saline Solution Retention Capacity [g/g] Initial PhaseIntermediate Phase Final Phase Example 1 35.1 34.8 35.2 Comparative 40.934.8 34.2 Example 1

As shown in Table 1, for Example 1, no dispersion is present in thenormal saline solution retention capacity among the initial phase, theintermediate phase, and the final phase and, for Comparative Example 1,dispersion occurs in the normal saline solution retention capacity amongthe initial phase, the intermediate phase, and the final phase. ForExample 1, no adhesion of the polymerization product of acrylic acid tothe strainer was observed while, for Comparative Example 1, adhesion ofthe polymerization product of acrylic acid was observed for 80% of thestrainers. The initial phase, the intermediate phase, and the finalphase are the timings of taking out the water-absorbing resin particlesfrom the treatment container, and the treatment time period and theconditions for the surface cross-linking treatment for thewater-absorbing resin particle precursor were substantially same.

It can be considered that, in Example 1, the surface cross-linking agentwas uniformly mixed with the surface of the water-absorbing resinparticle precursor, the surface cross-linking agent was uniformlypresent in the vicinity of the surface of each of the water-absorbingresin particles, uniform surface cross-linking was conducted, nodispersion was thereby present in the normal saline solution retentionscapacity among the initial phase, the intermediate phase, and the finalphase, and a product with constant physical properties was acquired.

It can be considered that, in contrast, in Comparative Example 1, thesurface cross-linking agent was not uniformly mixed with the surface ofthe water-absorbing resin particle precursor, and, in the initial phase,the surface cross-linking agent was little in the vicinity of thesurface of each of the waster-absorbing resin particles. It can beconsidered that, in the initial phase, the surface cross-linking wastherefore weakly conducted and the normal saline solution retentioncapacity was therefore high while the physical properties such as theliquid passage property and the gel strength were degraded. It can beconsidered that, on the contrary, in the final phase, the surfacecross-linking agent was much in the vicinity of the surface of each ofthe waster-absorbing resin particles compared to that of the initialphase. It can be considered that, in the final phase, the surfacecross-linking was therefore strongly conducted compared to that of theinitial phase and the physical properties such as the liquid passageproperty and the gel strength were improved while the normal salinesolution retention capacity was degraded. As above, in ComparativeExample 1, the dispersion occurred in the normal saline solutionretention capacity compared to that of Example 1 and no product withconstant physical properties was acquired.

From the above, it was confirmed that, in Comparative Example 1, some ofthe unreacted monomers remaining in the water-absorbing resin particlesvaporized to adhere to the strainer that was incorporated in the spraynozzle and the heat was transmitted thereto to cause polymerizationthereof to thereby cause the blocking out in the strainer to occur. Itcan be seen that this blocking out of the strainer degraded the functionof the spray nozzle and the surface cross-liking agent was therebyunable to be uniformly supplied to the surface of the water-absorbingresin particle precursor.

As above, as in Example, according to this disclosure, at least onefilter (the strainer) is present between the spray nozzle that sprays aliquid substance to the water-absorbing resin particle precursor and thetank that supplies the liquid substance, and the temperature of thefilter (the strainer) was controlled to be preferably 0 to 40° C., morepreferably 14° C. to 40° C., further preferably 15° C. to 30° C., andfurther more preferably 15° C. to 25° C. Any clocking out of the spraynozzle can thereby be prevented and a product with constant physicalproperties can stably be produced. The temperature of the Y-shapedstrainer of Example 1 was 17° C. and the temperature of the 100-meshstrainer incorporated in the spray nozzle of Comparative Example 1 was100° C. It can be considered that the temperature of the 100-meshstrainer of Comparative Example 1 was a high temperature compared tothat of the Y-shaped strainer of Example 1, and the polymerization ofthe adhering substance of the unreacted monomers was thereby furtheradvanced to cause the blocking out of the strainer.

Properly combining any optional embodiments with each other, of theabove various embodiments can cause each of the optional embodiments toachieve the effect to be achieved thereby.

This disclosure fully describes in relation to the preferred embodimentwith reference to the accompanying drawings while various deformationsand various modifications are obvious for those skilled in the art. Itshould be understood that these deformations and modifications areencompassed in this disclosure without departing from the scope of thisdisclosure stipulated in the appended claims.

When the apparatus for producing water-absorbing resin particles of thisdisclosure is used, any blocking out of the spray nozzle can besuppressed. Sufficient and uniform surface treatment is thereforeenabled for the surface of the water-absorbing resin particle precursorand water-absorbing resin particles having stable quality can beproduced.

EXPLANATIONS OF LETTERS OR NUMERALS

-   1 treating apparatus (apparatus for producing water-absorbing resin    particles)-   2 tank-   3 pump-   4 pure water supply line-   5 nitrogen supply line-   10 treatment container-   11 input entrance-   12 discharge exit-   20 stirring device-   21 stirring board-   22 rotational shaft-   23 rotation driving device-   30 heating device-   31 heat transmitting part-   32 heating jacket-   40 spray device-   41 supply line-   42 spray nozzle-   43 first pipe-   44 second pipe-   45 strainer-   51 spray exit-   52 chip part-   53 entrance-   54 main body part-   55 cap part

The invention claimed is:
 1. A method for producing water-absorbingresin particles, the method comprising a surface cross-linking treatmentbeing conducted by spraying a surface cross-linking agent to awater-absorbing resin particle precursor and heating the agent and theprecursor, the water-absorbing resin particle precursor includingunreacted monomers, wherein the surface cross-linking treatment isconducted by an apparatus for producing water-absorbing resin particles,and the apparatus includes: a treatment container in which the surfacecross-linking treatment is conducted; a stirring device including astirring member disposed in the treatment container; a heating devicethat heats an inside of the treatment container; a decompressing devicethat reduces a pressure in the treatment container; and a spray nozzledisposed in the treatment container, the spray nozzle spraying into thetreatment container the surface cross-linking agent, in a flow path inthe spray nozzle spanning from an entrance of the spray nozzle to aspray exit, a point whose opening cross-section is smallest in a flowpath through which a fluid passes is the spray exit.
 2. The method forproducing water-absorbing resin particles according to claim 1, theapparatus comprising a strainer disposed in a supply pipe on an upstreamside of the entrance of the spray nozzle, the strainer including pluralopenings each having an opening cross-section smaller than an openingcross-section of the spray exit of the spray nozzle.
 3. The method forproducing water-absorbing resin particles according to claim 2, whereinthe heating device is a heating jacket arranged on an outercircumference of the treatment container, and the strainer is disposedin the supply pipe on an outer side of the heating jacket.
 4. The methodfor producing water-absorbing resin particles according to claim 2,wherein the supply pipe for the surface cross-linking agent includes afirst pipe that is a header pipe having plural spray nozzles connectedthereto and a second pipe that connects the surface cross-linking agentsupply source and the first pipe to each other, and the first pipe andthe plural spray nozzles are arranged inside the treatment container,and the strainer is disposed in a portion on an outer side of thetreatment container, of the second pipe.
 5. The method for producingwater-absorbing resin particles according to claim 2, the apparatusfurther comprising a cooling device that cools the strainer.
 6. Themethod for producing water-absorbing resin particles according to claim2, wherein a nitrogen supply pipe is connected to the supply pipe. 7.The method for producing water-absorbing resin particles according toclaim 1, wherein the heating device includes a heat transmitting partthat is arranged in an inside of the stirring device.
 8. The method forproducing water-absorbing resin particles according to claim 2, whereinthe strainer is disposed on a downstream side of a pump which isdisposed in the supply pipe to supply the surface cross-linking agent tothe spray nozzle through the supply pipe.
 9. A method for producingwater-absorbing resin particles, the method comprising: spraying asurface cross-linking agent to a water-absorbing resin particleprecursor in a treatment container, the water-absorbing resin particleprecursor including unreacted monomers; stirring the water-absorbingresin particle precursor in the treatment container; heating the surfacecross-linking agent and the water-absorbing resin particle precursor inthe treatment container; and reducing a pressure in the treatmentcontainer, wherein the surface cross-linking agent is sprayed into thetreatment container through a spray nozzle disposed in the treatmentcontainer, in a flow path in the spray nozzle spanning from an entranceof the spray nozzle to a spray exit, a point whose opening cross-sectionis smallest in a flow path through which a fluid passes is the sprayexit.